A THESIS
THE DESIGN OF A
TESTING TRANSFORMER
SECONDARY VOLTAGE- 60,000.
PRIMARY VOLTAGE- HO.
CAPACITY 3 K.W.
WITH
DRAWINGS
AND
RESULTS OF TESTS
ON DIELECTRIC STRENGTH
OF VAR I OUS
INSULATORS.
5. H. Graf.
ci*
Theory of the Transformer
An alternating current transformer is an apparatus designed for the purpose of either raising or lowering the voltage
of an alternating current circuit.
In general it consists of
two electrical circuits interlinked with a magnetic circuit, one
called the primary into which the electrical energy is fed, and
the ether called the secondary from which the electrical energy
is taken.
The operation of a transformer depends upon the law of
induction;
i,e, that a change in the rate of cutting lines of
force by a conductor induces in the conductor an electro motive
force which varies as the number of conductors cutting lines of
force and the rate of cutting them.
Since the current used in
the transformer is alternating, the number of lines of force or
the flux changes from zero to a positive maximum, through zero
to a negative maximum and back to zero again, thus furnishing
the necessary change in the number of lines of force out for the
primary to induce an electro motive force in the secondary.
Since the total flux of the magnetic circuit is dependent upon the number of turns of wire in the primary circuit, it
follows that if the number of turns is not the same in the
primary and secondary circuits, the electro motive forces in the
t7o circuits will not be of the sane potential.
This fact is Lade
use of in the transformer and a ratio, called the ratio of transformation, is established between the primary and secondary
circuits.
If the secondary is of a higher potential than the
primary, the transformer is called a step up transformer;
if it
is of a lower potential, the transformer is called a step down
transformer.
Step up transformers have their chief use in
generating plants where, owing to the limitations of the generators
a higher potential is needed for economical transmission.
How-
ever, for distribution, the potential must be reduced for practical
reasons, hence step down transformers are employed to reduce the
pressure so it may be safely used.
These are the general uses of the transformer as applied
to the distribution of electrical energy for commercial purposes.
It has many special applications one of which is in testing the
insulating properties of dielectrics.
For this purpose it is un-
excelled, as the extremely high voltages necessary for the breaking
down of certain dielectrics render it almost impossible and quite
impracticable for them to be generated at the dynamo.
In making extensive tests, voltages as high as 500,000
are frequently used and in one case 1,000,000 has been reached.
To develop these high voltages, the testing transformer is used.
In principle it is the same as any other transformer, and differs
in construction only in that to withstand the enormous differences
of potential, it must be more perfectly insulated.
Then also
instead of coupling its secondary to a line, its terminals are
brought out to binding pests much the same as an induction coil
and are here connected to a suitable spark gauge.
Since it is
known ho,,- far a given voltage will cause a spark to pass in air,
the length of the air gap forms a means of determining the voltage used on the dielectric.
The tendency at present is toward increasing the voltages
used in the transmission of electrical energy for power and light
ing purposes.
Thi is due to the fact that the line loss is less
,
for high voltages than for low ones since the current is less for
a given power transmitted and the loss varies as the square of the
current.
To meet the increasing voltages, there is a demand for
more perfect insulators to be used both in the line and at the
machine.
To gain any idea of the insulating value of a di electric
when subjected to the constant strain of a high potential, it is
necessary to subject it to a brief strain of much higher potential
perhaps twice as high.
For obtaining the necessary high pressure
in making the tests the testing transformer is used.
Design.
The following is the deign of a typical testing transformer of 3 K.W. capacity, core type to step 110 volts up to
60,000.
In this work Ep, will mean the impressed primary E.M.F.
Bm, the maximum flux density;
primary circuit;
Tp, the number of turns in the
t, the depth of the secondary coil in inches;
al the length of the core limbs;
limbs;
b, the distance between the core
e, and d, the dimension of the cross section of the cores
which are to be square, hence o = d;
diameter if the secondary wire.
The known values if these are
assumed as follows, Ep =110 volts;
Tp = 82 turns;
f 7 60 cycles.
f, the frequency, and d, the
Bm = 14,000 gausses;
The secondary wire is to be No.35
hence from a wire table d = .01162 in.
a, b, o, and t, are to be
calculated.
From "Sheldon's Alternating Current Machines", the
necessary area of the core A, is found by the following equation..
A =
10 8 Ep
1.417Tf Tp Bra
Substituting in this formula the values assumed and it
becomes.
10 8 X 110
1.41-X 3.1416 X 82 X 14,000
=
36 sq.
om.
This gives us a core 6 cm. square or reducing to inches
2 3/8 in.
The high flux density was taken because in making insulation tests, the secondary becomes practically short circuited
and the high flux density causes a high leakage flux and a smaller
lose f current. For an ordinary lighting or power transformer,
the flux density for a 60 cycle current would not exceed 9,000
gausses.
Working at a higher density lowers the efficiency some-
what but this is a matter of no great importanoe in an instrument
designed only for test purposes.
It is intended in this instrument that the primary and
secondary -coils be wound on the separate limbs of the core.
The
secondary will be the larger, hence the dimensions a, b, and t,
will be derived from calculations based on the secondary coil only.
Let the ratio of a to b, be as 1 to 1.5; and
let 15/16 in. be allowed for clearance on each coil.
Then it is
evident that in order to assemble the transformer, b must equal
2(t
15/16).
To determine t, if 11/16 in. be allowed at each end of
the coils for insulation, and taking the ratio of transformation
as 550, we have the expressioR,
O4
550 Tp
t= a - 1 3/8
Now since a = 1.5 b, and b = 2(t f 15/16) we may substin
tute in the expressio for t, and it9.then becomes,
t = 3(t
550 X 82 X .01162'"
15/16) - 1 3/8.
=
1 7/8 inches.
Substituting this value of t, in the expression for b,
and we have
b = 2(1 7/8 f 15/16) = 5 5/8 in. and a, which equals
1.5 b, equals 8 3/8 in.
To obtain the volume of the iron in the core in cubic
centimetres in order to oompute the hystersis loss we assume that
90% of the space occupied by the laminations is metal.
This gives V = 2(a t b
2o) X o2 X 2.54 X .9
.
Substituting the
values found in the equation it becomes,
V= 2(8 3/8
5 5/8 t 4 3/4) X 2 3/82 X 2.54 X .9
Solving this and we get for the vale of V
3,120 cu. am.
To obtain the loss due to hystersis Ph, we
apply Steinmetes Law and assume the constant n = .003.
Ph = .003/107XfXVXBml°6
Substituting the values already found in this expression
we have,
Ph = .003/107 X 60 X 3,120 X 1400001 8 = 174 watts.
To determine the loss due to Foucalt currents Pf, we
make use of an empirical formula, Pf = 108V(y7X f X Bm)
where y is the thickness of one plate.
2
This was taken as .01 in.
Then Pf equals 10-8 X 3,120(.01 X 60 X 14,000)2 = 22.2 watts.
Since the o pacity of this transformer is 3 K.W. then the
full load primary current will be
3000/110 or 27.25 amperes, and
the secondary current will be, disregarding losses 27.25/550 or
.05 amperes.
The primary is designed to be wound with 2 No.8
wires in 7arallel and its 82 turns will have a resistance of
.03138 ohms.
The secondary is to be wound with No.35 and it will
have a resistance of 24,087 ohms.
Letting Es, be the secondary E.H.F. Is, be the secondary
current.
Rp, be the primary resistance, and Rs, be the secondary
resistance, we have the following equation to give the efficiency
in per cent e, at any load.
Es X Is
e= (Fe X Is) t Ph t Pf f 102 Rs t T550 ITY2TRT--
Substituting the values in the equation we find the
efficiency to be,
601000 X .05
e
_§01000 X .05 t 174 t 22.2 rf (24,00027-4---tt tzra-rwmge
Solving e equals .934 X 100 or 95.4 la
Introduction to Drawings.
The following drawings show the actual construction of
the transformer and are intended for a working set.
Plate A. shows the general construction of the ooze
which is built up
of soft electrical iron.
It will be noticed
that the core limbs are held together by wrought iron platew en
either side through which pass stove bolts.
The limbs are firmly
clamped to the yokes by two steel bolts passing through angle
irons which hold the yokes together.
The connections between
the yokes and limbs must be plane and smooth in order to make a
perfect joint. Plata 7A. also shows the method of securing the
transformer to the case cover by means of four holders.
This is
again shown in Plate D.
plate B. shows the details of the core, the plates for
holding the limbs together, the angle irons, and the necessary
bolts.
Plate C. is a general view of the coils.
It willbe
noticed that the secondary is to be wound in 36 sections and that
the terminals are to be led through heavy rubber tubes to avoid
short circuiting on the case.
Plate D. is a general drawing showing dimensions of the
case and the method of holding the transformer in the case.
This
plate also shows the sheet iron tank for containing transformer
oil in which the transformer is submerged.
This tank is to be
an exact fit for the outer case
Plate E. is ageneral view of the spark gauge and also
shows tte details of the parts.
It will be noticed that the
gauge is mounted on convoluted. hard rubber standard
.
It is de-
signed that the spark pass between needle points which must be
kept sharp. The micrometer reads to .001 in thus giving very
accurate results.
Plate F. is an oblique projection of the complete
transformer, and Plate G. is a diagram of the connections show
ing the manner of connecting the transformer up to make a test.
Drawings for-
TESTING
300o
(1'
6o
Naffs
Cycles
Volts Sec. o,o o o
TRANSFORMER
Type
Volts
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Method of Making Tests.
the 2ample to be
In making tests with the transformer,
surfaces, which are cell
tested is placed flat between two metallic.
A water
netted with the high tension side of the transformer.
the purpose of regulating
rheostat is put in the primary circuit for
the impressed voltage.
at a minimum distano 0
The needle points are then adjusted
material under test, and
apart, depending of course upon the
passes between the
the impressed voltage raised until a spark
and the operThe air gap is then increased in length,
points.
down and the spark passes
ation repeated until the sample breaks
voltage is then
through it instead of the air.. The break -down
last position
taken from a table or curve corresponding to the
of the needle points.
beyond the edges
The sample should project considerably
Owing to the surface leakage a
of the compressing surfaces.
of the surface of
spark will pass over a much greater distance
an insulator than it will in air.
whether a given
The apparatus may be used to determine
impressed electro motive
sample of insulation will withstand an
The air gap is set so as to repforce without breaking down.
voltage, and the sample subresent the value of the prescribed
spark aorcias the
jected to the pressure which maintains the
In case of a break down the spark will cease.
gap.
be desired,
Should the temperature of the dielectric
be used so that the
some suitable means of heating it should
temperature may be measured.
SPARKING DISTANCES IN AIR
et we Cr?
en sharp Ne ecile Points for-
//, clie sinusoidal Voltages
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/Ca/ Er,yin e ers.
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Results of Tests
in
The following results were obtained from tests made
small transformer of about
the manner described, and by means of a
voltage of 10,000.
1 K.V.A. capacity, giving a maximum secondary
Several sambles of each material were testeo and an
computed for each
average puncture voltage per mil thickness was
Although lack of time and the small. capacity of the
dielectric.
tests, we belie,!e
apparatus did not permit an extended series of
that the results below are
some practical value.
Perrino, in his work "Electrical Conductors", says;"It
the protection offered by
is not possible to state distinctly that
trials
an insulation varies directly with its thickness, indeed,
their
involving the piercing of dielectrics seem to inJ'ioate that
to the suare than to
resisting power is proportional more nearly
though in insulating wires
the first power of their thickness;
is unsafe to assume
used for transmitting alternating currents it
than the first
that the resisting power increases more rapidly
account of a general uncertainty conpower ofthe thickness on
motive force to the
cernin6 the relation of the maximum electro
in the forms of
average value on account of great variations
different electro motive force curves generated by commercial
machines".
To give the break down voltage per mil has therefore
been demed suficient.
Trade Name.
Break Down
Voltage.
Per Mil.
Fiberoid.
Remarks.
Manufacturer.
Color - Gray
Wilmington Fibre
Specialty Co.
Wilmington, Del.
142
1 & 2.
188
1 & 2.
42
1 & 2.
129
1 & 2.
nnnn tt
White Fibre.
Red Fibre.
It
Red Fibre.
n
11
140
ft
it
If
it
4.
74
Extra Flexible
Micanite Cloth.
ft
IT
From O.A.C. Stock.
Oiled Linen.
Extra Flexible
Micanite Paper.
II
Partly 4.
n
Mica Insulator Co.
Chicago, Ill.
Ty
it
it
It
It
ft
it
it
It
tt
It
II
It
n
it
It
ft
It
It
it
it
it
n
It
ft
It
ft
"
"
"
"
"
it
132
tt
Micanite Paper.
ft
148
Empire Cloth
No.5.
858
4.
925
4,
512
4.
Empire Cloth
No.7.
Empire Cloth
Nb.8.
Empire Paper
No.D.
870
Dark Amber Mica
Brantford, Ont.
North Carolina
White Mica.
Somewhat 3.
4
The Cawood Mica Co.
Buffalo, N.Y.
1560
2.
n
1290
'I
n
n
n
2.
1. Absorbs moisture slightly.
3. Mechanically weak.
2. Fire resisting.
4. Combustible.
The above tests were all made at ordinary room temperature.
Cloth and Paper
Some of the heavier grades of Empire
were beyond the capacity of our transformer.
All the material
from the Cawood
tested was from commercial samples, the mica
North Carolina.
Mica Co. was taken from mines in Ontario and
Brantford, Ont.
The first piece coming from Hughes' Mine near
Shelby, N.C.
and the second from the Hefner Mine near